The problem of developing a masking tape having good heat resistance is discussed in a general way in Satas: "Handbook of Pressure-Sensitive Adhesive Technology", Van Nostrand Reinhold Co., 1982, pages 203-207. After noting that "conventional natural rubber adhesives" do not have the required temperature resistance for a masking tape, the handbook says: "To impart the required degree of heat resistance to the adhesive, it is usual to effect a degree of crosslinking to the natural rubber. We have seen earlier that the tack and adhesion of pressure-sensitive adhesives depend upon the viscoelastic properties of the rubber-resin blend. Consequently, the degree of crosslinking has to be carefully controlled; if the degree of curing is too high, then the adhesive will lose its ability to deform readily and hence wet the surface to which it is being applied, but it has to be sufficiently cured to have resistance to the paint solvent, and also not to be softened too much by the stoving cycle" (page 203).
As pointed out in U.S. Pat. No. 3,922,464 (Silver et al.): "Early pressure-sensitive adhesives were based on organic solvent solutions of natural or synthetic rubber tackified by some type of resinous material. These so called `rubber-resin` adhesives are still widely used in masking tape, one major reason being that they can be formulated so as not to display adhesion buildup, i.e., their adhesion to the membrane does not increase to the point where the tape cannot be removed cleanly, even after exposure to the heat of a drying or curing oven." (col. 1, lines 40-48). The Silver patent then alludes to problems of batch-to-batch consistency of rubber-resin adhesives and to staining problems, indicating that these problems are avoided by a wholly synthetic adhesive system based on a copolymer of (a) vinyl monomer such as iso-octyl acrylate, (b) a small amount of vinyl-unsaturated, homopolymerizable emulsifier monomer which is a surfactant such as sodium-2-sulfoethylmethacrylate, and preferably (c) a small amount of zwitterion monomer having a free radically polymerizable unsaturated group. The Silver patent also recommends that the adhesive layer contain a small amount of external emulsifier such as sodium dodecyl benzene sulfonate.
The Silver patent reports "the absence of adhesion increase, or `buildup` when tape made with such an adhesive is applied to an enameled or lacquered surface. The adhesion stability is observed even when the tape, after application, is subjected to temperatures of 250.degree. F. (about 120.degree. C.) for 1 hour or for longer periods of time at lower temperatures . . . " (col. 3, lines 2-10). Preferred tapes of the Silver patent have been cleanly removed from enameled or lacquered surfaces after one hour at 150.degree. C., a temperature commonly used in automotive painting.
In order to reduce solvent emissions, the automotive industry is extensively converting to basecoat/clearcoat paint systems (usually acrylic enamel systems) to which presure-sensitive adhesives are less adherent. Preferred tapes of the silver patent would not remain well adhered at 150.degree. C. when subjected to the sort of lifting forces that are encountered when a masking tape is applied to a curved surface and/or is used to hold an apron. Another widely used automotive paint involving lifting problems at 150.degree. C. of the Silver tapes and other prior masking tapes is "50J" acrylic enamel of Ford Motor Co.
Both the pressure-sensitive adhesive of the above-cited Silver patent and that of U.S. Pat. No. Re. 24,906 (Ulrich) are copolymers of nontertiary alkyl acrylates such as 2-ethylhexyl acrylate, but the Ulrich adhesive is a copolymer of such an acrylate and a small proportion of a highly polar monomer such as acrylic acid which produces adhesion buildup. Because of this, the Ulrich adhesive is not suitable for masking tapes, even though a continuation-in-part of the Ulrich reissue patent, U.S. Pat. No. 3,008,850, which concerns acrylates copolymerized with small proportions of acrylonitrile or methacrylonitrile, reports that a strip of the pressure-sensitive adhesive tape of Example 1 was cleanly stripped from an enameled automotive panel after 30 minutes at 121.degree. C.
U.S. Pat. No. 4,418,120 (Kealy et al.) states: "It has long been recognized that adhesives that consist essentially of a copolymer of alkyl acrylate and a minor proportion of copolymerizable monomer such as acrylic acid do not require a tackifying resin and are able to resist aging; thus, such adhesives have advantages over the earlier and more traditional rubber-resin adhesives; cf. Ulrich U.S. Pat. No. 24,906" (col. 1, lines 54-60). The Kealy patent concerns the need to improve both the adhesion and the heat resistance at 70.degree. C. of such an acrylate:acrylic acid adhesive to low energy surfaces, and does this by employing a tackifying resin for better adhesion and by crosslinking the tackified pressure-sensitive adhesive for better heat resistance at 70.degree. C. The Kealy patent does not suggest that its tapes would be removable. Neither do other patents and publications which mention that tackifiers can be used with acrylate adhesives. For example, see U.S. Pat. Nos. 3,740,366 and 4,077,926 (Sanderson).
Also, see page 89 of Jones: "Elastomers and Resin Modifiers for Water-based Adhesives", Proceeding Today's Technology, Holiday Inn O'Hare, Rosemont, IL., June 23-24, 1981, which notes that copolymers of the Ulrich reissue patent "have typically been used without tackifying resins or with quite low levels" but that tackifying resins can "contribute significantly to probe tack, peel and quick stick."
U.S. Pat. Nos. 3,931,087 and 4,012,560 (Baatz et al.) concern pressure-sensitive adhesive tape based on a copolymer like those of the above-cited Silver patent except employing a specific sulfonic acid as the emulsifier monomer. The specification of each of the Baatz patents closes with a paragraph which says that the invention contemplates the use of various modifiers in the pressure-sensitive adhesive resins, including tackifiers. The function of those tackifiers is not mentioned.